A tunneling magnetoresistance or magnetoresistive (TMR) reader is a type of magnetic sensor that utilizes a magnetic tunnel junction (MTJ). In general, a TMR reader includes two ferromagnetic layers that are separated by a barrier layer (e.g., MgO). In a current-perpendicular-to-plane (CPP) geometry type TMR structure, the resistance of the structure for a current flowing perpendicular to the barrier layer depends on the relative angle between the magnetization of both ferromagnetic layers. The quality of such a TMR device is characterized by its magnetoresistance ratio (dR/R) that indicates the magnitude change in resistance of a material due to an external magnetic field. The TMR device exhibits magnetoresistance at the magnetic tunnel junction. Generally, the magnetoresistance ratio decreases with both increasing temperature and increasing bias voltage.
The signal-to-noise ratio (SNR) of the TMR reader depends on both the signal (which is proportional to amplitude and thus dR/R of the thin film stack of the TMR reader) and noise. The noise can be partitioned into magnetic noise and electrical noise. While the magnetic noise is correlated to the magnitude of the signal, the electrical noise is proportional to the resistance-area (RA) product of the device. Therefore, the SNR of the TMR reader may be increased by increasing the signal (e.g., increasing the magnetoresistance ratio dR/R of the film stack) and/or reducing the noise (e.g., reducing the RA to decrease electrical noise). In a generally known TMR structure, however, dR/R will typically decrease when RA is reduced (e.g., due to wave function overlap because of the reduced barrier thickness as well as increase in interlayer coupling).
In the related art, in situ-heating has been used after the deposition of the tunnel barrier (e.g., a MgO tunnel barrier layer) to improve the dR/R for a given RA. This is done to improve the crystallinity of the MgO tunnel barrier layer before deposition of the next layer. However, it is desirable to further improve the SNR performance of a TMR device (e.g., a reader).